MRI center point artifact elimination using realtime receiver phase control

ABSTRACT

A method and apparatus for eliminating baseline error artifacts in NMR images produced using ultrafast pulse sequences including alternating the phase of a reference signal during the reception of NMR echo signals to invert alternate views in the acquisition. The inverted views are re-inverted prior to image reconstruction such that any dc levels introduced during acquisition are converted to a high spacial frequency which moves the resulting artifacts to the borders of the image reconstructed using a Fourier transformation.

BACKGROUND OF THE INVENTION

The field of the invention is nuclear magnetic resonance imaging methodsand systems. More particularly, the invention relates to the removal ofbaseline error artifacts in images produced by ultrafast imagingmethods.

When a substance such as human tissue is subjected to a uniform magneticfield (polarizing field B₀), the individual magnetic moments of thespins in the tissue attempt to align with this polarizing field, butprecess about it in random order at their characteristic Larmorfrequency. If the substance, or tissue, is subjected to a magnetic field(excitation field B₁) which is in the x-y plane and which is near theLarmor frequency, the net aligned moment, M_(z), may be rotated, or"tipped", into the x-y plane to produce a net transverse magnetic momentM_(t). A signal is emitted by the excited spins, and after theexcitation signal B₁ is terminated, this NMR signal may be received andprocessed to form an image.

When utilizing these signals to produce images, magnetic field gradients(G_(x) G_(y) and G_(z)) are employed. Typically, the region to be imagedis scanned by a sequence of separate measurement cycles (referred to as"views") in which these gradients vary according to the particularlocalization method being used. The resulting set of received NMRsignals are digitized and processed to reconstruct the image using oneof many well known reconstruction techniques.

A well known problem with MRI systems is the introduction of baselineerrors into the received NMR echo signals. This error occurs when a dclevel is added to the NMR echo signal during its reception, demodulationand digitization. This dc level may be introduced by dc offsets in theanalog receiver circuitry, or by stray signals that are demodulated to adc level along with the NMR echo signal. Continued improvements to thereceiver hardware have reduced, but not eliminated this problem.

One way to eliminate baseline errors is to acquire two NMR echo signalsat each phase encoding view. The phase of the RF excitation pulse isinverted 180° for one of the two pulse sequences, and the two receivedNMR echo signals are subtracted. The subtraction nulls the dc levelintroduced by the receiver and doubles the level of the NMR signal forthat view. Unfortunately, this solution also doubles the scan timebecause two measurements are required of each phase encoding view.

A similar solution which also addresses the increased scan time wasdisclosed in U.S. Pat. No. 4,612,504, issued Sep. 16, 1986 entitled"Method For Removing The Effects Of Baseline Error Components In NMRImaging Applications". This method alternates the phase of the RFexcitation pulse for successive phase encoding views. Prior to imagereconstruction, the NMR signals for alternate phase encoding views arere-inverted so that the NMR signals in all views have the same polarity.This re-inversion also inverts any dc level in alternate views. As aresult, the dc level alternates in polarity for successive views ink-space and is now a high frequency signal component in the phaseencoding direction. The subsequent column Fourier transformation usedduring image reconstruction translates this high frequency component andplaces an artifact at the edges of the reconstructed image rather thanits center. This is the prevailing method for solving the baseline errorproblem, and it is very effective.

The concept of acquiring NMR image data in a short time period has beenknown since 1977 when the echo-planar pulse sequence was proposed byPeter Mansfield (J. Phys. C.10: L55-L58, 1977). In contrast to standardpulse sequences, the echo-planar pulse sequence produces a large numberof NMR echo signals for each RF excitation pulse. These NMR signals canbe separately phase encoded so that an entire scan of 64 views can beacquired in a single pulse sequence of 20 to 100 milliseconds induration. The advantages of echo-planar imaging ("EPI") are well-known,and there has been a long felt need for apparatus and methods which willenable EPI to be practiced in a clinical setting. Other echo-planarpulse sequences are disclosed in U.S. Pat. Nos. 4,678,996; 4,733,188;4,716,369; 4,355,282; 4,588,948 and 4,752,735.

A variant of the echo planar imaging method is the Rapid AcquisitionRelaxation Enhanced (RARE) sequence which is described by J. Hennig etal in an article in Magnetic Resonance in Medicine 3,823-833 (1986)entitled "RARE Imaging: A Fast Imaging Method for Clinical MR." Theessential difference between the RARE sequence and the EPI sequence liesin the manner in which echo signals are produced. The RARE sequenceutilizes RF refocused echoes generated from a Carr-Purcell-Meiboom-Gillsequence, while EPI methods employ gradient recalled echoes.

Yet another variant is described by D. A. Feinberg and K. Oshio "GRASE(Gradient and Spin Echo) MR Imaging: A New Fast Clinical ImagingTechnique", in Radiology, 181:597-604, 1991.

All of these "ultrafast" imaging methods involve the acquisition ofmultiple echo signals from a single excitation pulse in which eachacquired echo signal is separately phase encoded. Each pulse sequence,or "shot," therefore results in the acquisition of a plurality of views.In a single shot acquisition, the method previously used to eliminateimage artifacts due to baseline errors cannot be used with theseultrafast pulse sequences, because there is no one-for-onecorrespondence between RF excitation pulse and NMR echo signal.

SUMMARY OF THE INVENTION

The present invention is a method for eliminating baseline errorartifacts in NMR images produced using ultrafast pulse sequences. Morespecifically, when performing an ultrafast pulse sequence in which aplurality of NMR echo signals are produced following the application ofa single RF excitation pulse, the present invention is practiced byalternating the polarity of the successive phase encoded NMR echosignals during their reception. Prior to image reconstruction, theinverted NMR echo signals are reinverted with the result that any dclevel introduced during reception is inverted in alternate phaseencoding views and becomes a high frequency component. During imagereconstruction, any artifacts therefrom are then displayed at the phaseencoding borders of the reconstructed image.

A general object of the invention is to eliminate artifacts due tobaseline errors without increasing the scan time. By alternating thepolarity of the received NMR echo signals in a pattern that results inthe inversion of alternate views in k-space, a proper image isreconstructed by first re-inverting alternate views in k-space. There-inversion also inverts any dc level produced during reception and theresulting high frequency of this component is transferred to the phaseencoding border of the reconstructed image during the subsequent Fouriertransformation along the phase encoding direction.

A more specific objective is to reduce artifacts due to baseline errorswithout costly hardware changes or burdensome changes to the MRI systemsoftware. Inversion of the received NMR echo signals is easilyaccomplished by inverting the reference signal used during demodulationof the received echo signals. Such an inversion is controlled by thesame MRI system components and software used to execute the fast spinecho pulse sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an MRI system which employs the presentinvention;

FIG. 2 is an electrical block diagram of the transceiver which formspart of the MRI system of FIG. 1; and

FIG. 3 is a graphic representation of an EPI pulse sequence which isemployed in the preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, there is shown the major components of apreferred MRI system which incorporates the present invention. Theoperation of the system is controlled from an operator console 100 whichincludes a keyboard and control panel 102 and a display 104. The console100 communicates through a link 116 with a separate computer system 107that enables an operator to control the production and display of imageson the screen 104. The computer system 107 includes a number of moduleswhich communicate with each other through a backplane. These include animage processor module 106, a CPU module 108 and a memory module 113,known in the art as a frame buffer for storing image data arrays. Thecomputer system 107 is linked to a disk storage 111 and a tape drive 112for storage of image data and programs, and it communicates with aseparate system control 122 through a high speed serial link 115.

The system control 122 includes a set of modules connected together by abackplane. These include a CPU module 119 and a pulse generator module121 which connects to the operator console 100 through a serial link125. It is through this link 125 that the system control 122 receivescommands from the operator which indicate the scan sequence that is tobe performed. The pulse generator module 121 operates the systemcomponents to carry out the desired scan sequence. It produces datawhich indicates the timing, strength and shape of the RF pulses whichare to be produced, and the timing of and length of the data acquisitionwindow. The pulse generator module 121 connects to a set of gradientamplifiers 127, to indicate the timing and shape of the gradient pulsesto be produced during the scan. The pulse generator module 121 alsoreceives patient data from a physiological acquisition controller 129that receives signals from a number of different sensors connected tothe patient, such as ECG signals from electrodes or respiratory signalsfrom a bellows. And finally, the pulse generator module 121 connects toa scan room interface circuit 133 which receives signals from varioussensors associated with the condition of the patient and the magnetsystem. It is also through the scan room interface circuit 133 that apatient positioning system 134 receives commands to move the patient tothe desired position for the scan.

The gradient waveforms produced by the pulse generator module 121 areapplied to a gradient amplifier system 127 comprised of G_(x), G_(y) andG_(z) amplifiers. Each gradient amplifier excites a correspondinggradient coil in an assembly generally designated 139 to produce themagnetic field gradients used for position encoding acquired signals.The gradient coil assembly 139 forms part of a magnet assembly 141 whichincludes a polarizing magnet 140 and a whole-body RF coil 152. Atransceiver module 150 in the system control 122 produces pulses whichare amplified by an RF amplifier 151 and coupled to the RF coil 152 by atransmit/receive switch 154. The resulting signals radiated by theexcited nuclei in the patient may be sensed by the same RF coil 152 andcoupled through the transmit/receive switch 154 to a preamplifier 153.The amplified NMR signals are demodulated, filtered, and digitized inthe receiver section of the transceiver 150. The transmit/receive switch154 is controlled by a signal from the pulse generator module 121 toelectrically connect the RF amplifier 151 to the coil 152 during thetransmit mode and to connect the preamplifier 153 during the receivemode. The transmit/receive switch 154 also enables a separate RF coil(for example, a head coil or surface coil) to be used in either thetransmit or receive mode.

The NMR signals picked up by the RF coil 152 are digitized by thetransceiver module 150 and transferred to a memory module 160 in thesystem control 122. When the scan is completed and an entire array ofdata has been acquired in the memory module 160, an array processor 161operates to Fourier transform the data into an array of image data. Thisimage data is conveyed through the serial link 115 to the computersystem 107 where it is stored in the disk memory 111. In response tocommands received from the operator console 100, this image data may bearchived on the tape drive 112, or it may be further processed by theimage processor 106 and conveyed to the operator console 100 andpresented on the display 104.

Referring particularly to FIGS. 1 and 2, the transceiver 150 producesthe RF excitation field B1 through power amplifier 151 at a coil 152Aand receives the resulting NMR signal induced in a coil 152B. Asindicated above, the coils 152A and B may be separate as shown in FIG.2, or they may be a single wholebody coil as shown in FIG. 1. The base,or carrier, frequency of the RF excitation field is produced undercontrol of a frequency synthesizer 200 which receives a set of digitalsignals from the CPU module 119 and pulse generator module 121. Thesedigital signals indicate the frequency and phase of the RF carriersignal produced at an output 201. The commanded RF carrier is applied toa modulator and up converter 202 where its amplitude is modulated inresponse to a signal R(t) also received from the pulse generator module121. The signal R(t) defines the envelope of the RF excitation pulse tobe produced and is produced in the module 121 by sequentially readingout a series of stored digital values. These stored digital values may,in turn, be changed from the operator console 100 to enable any desiredRF pulse envelope to be produced.

The magnitude of the RF excitation pulse produced at output 205 isattenuated by an exciter attenuator circuit 206 which receives a digitalcommand, TA, from the backplane 118. The attenuated RF excitation pulsesare applied to the power amplifier 151 that drives the RF coil 152A. Fora more detailed description of this portion of the transceiver 122,reference is made to U.S. Pat. No. 4,952,877 which is incorporatedherein by reference.

Referring still to FIGS. 1 and 2 the NMR signal produced by the subjectis picked up by the receiver coil 152B and applied through thepreamplifier 153 to the input of a receiver attenuator 207. The receiverattenuator 207 further amplifies the signal by an amount determined by adigital attenuation signal (RA) received from the backplane 118.

The received signal is at or around the Larmor frequency, and this highfrequency signal is down converted in a two step process by a downconverter 208 which first mixes the NMR signal with the carrier signalon line 201 and then mixes the resulting difference signal with the 2.5MHz reference signal on line 204. As described above, the phase of thecarrier signal on line 201 is controlled by the frequency synthesizer200 in response to a phase command received from the pulse generatormodule 121. To practice the present invention, this phase command ischanged during the fast spin echo pulse sequence to invert (i.e. 180°phase change) the carrier signal on line 201 during the reception ofalternate ones of the NMR echo signals in the shot. Inversion of thecarrier signal on line 201 effectively inverts the NMR echo signal atthe output of down converter 208.

The down converted NMR signal is applied to the input of ananalog-to-digital (A/D) converter 209 which samples and digitizes theanalog signal and applies it to a digital detector and signal processor210 which produces 16-bit in-phase (I) values and 16-bit quadrature (Q)values corresponding to the received signal. The resulting stream ofdigitized I and Q values of the received signal are output throughbackplane 118 to the memory module 160 where they are employed toreconstruct an image.

The 2.5 MHz reference signal as well as the 250 kHz sampling signal andthe 5, 10 and 60 MHz reference signals are produced by a referencefrequency generator 203 from a common 20 MHz master clock signal. For amore detailed description of the receiver, reference is made to U.S.Pat. No. 4,992,736 which is incorporated herein by reference.

The echo-planar (EPI) pulse sequence employed in the preferredembodiment of the invention is illustrated in FIG. 3. An RF excitationpulse 250 is applied in the presence of a G_(z) slice select gradientpulse 251 to produce transverse magnetization in a slice. The excitedspins are rephased by a negative lobe 252 on the slice select gradientG_(z) and then a time interval elapses before the readout sequencebegins. A total of 128 separate NMR echo signals, indicated generally at253, are acquired during the EPI pulse sequence. Each NMR echo signal253 is a different view which is separately phase encoded to scan k_(y)-space in 128 monotonically acquired views centered about k_(y) =0. Thereadout sequence is positioned such that the view acquired at k_(y) =0occurs at the desired echo time (TE).

The NMR echo signals 253 are gradient recalled echoes produced by theapplication of an oscillating G_(x) readout gradient field 255. Thereadout sequence is started with a negative readout gradient lobe 256and the echo signals 253 are produced as the readout gradient oscillatesbetween positive and negative values. A total of 128 samples are takenof each NMR echo signal 253 during each readout gradient pulse 255. Thesuccessive 128 NMR echo signals 253 are separately phase encoded by aseries of 128 G_(y) phase encoding gradient pulses 258. The first pulseis a negative lobe 259 that occurs before the echo signals are acquiredto encode the first view at k_(y) =-64. Subsequent phase encoding pulses258 occur as the readout gradient pulses 255 switch polarity, and theystep the phase encoding monotonically upward through k_(y) space.

As the phase encoding is stepped through k_(y) space and the 128 NMRecho signals 253 are acquired, the phase command is alternated asindicated at 260. These alternations represent a 180° phase shift in thecarrier signal on line 201 (FIG. 2) and they cause the polarity ofalternate ones of the NMR echo signals 253 to be inverted. At thecompletion of the EPI pulse sequence, therefore, 128 separate frequencyencoded samples of 128 separately phase encoded (and alternatingpolarity) NMR echo signals 253 have been acquired. The resulting 128×128element array of complex numbers is an NMR data set which is used toreconstruct an image.

Prior to reconstructing the image, however, the polarity of alternaterows of samples in this NMR data set are re-inverted. Each row ofsamples represents one phase encoded NMR echo signal, and byre-inverting alternate rows, the polarity of all the acquired NMR echosignals are again the same. As explained above, however, thisre-inversion of alternate rows also inverts any dc level which wasintroduced into the NMR signals as they were acquired. Consequently, thedc level is now a signal component that alternates in polarity in thephase encoding direction of k-space (i.e. along the k_(y) axis of theNMR data set).

Image reconstruction is accomplished by performing a two-dimensionalFourier transformation on the altered NMR data set. As a naturalconsequence of the Fourier transformation performed in the columndirection (i.e. along the k_(y) axis), the high frequency alternating dccomponent is transposed to a location in image space on the imageborders (i.e. y=±64). Consequently, any resulting artifact caused bybaseline errors is far removed from the center of the reconstructedimage where it will not interfere with the diagnostic quality of theimage.

While the invention is described with respect to a single shot EPI pulsesequence, it should be apparent to those skilled in the art that it isequally applicable to odd-interleaved multishot EPI and to otherultrafast pulse sequences. For example, the RARE pulse sequence producesa plurality of NMR echo signals following the creation of transversemagnetization by a single RF excitation pulse. The polarity of these canbe alternated in the same manner as described above by changing thephase command prior to each NMR echo signal acquisition.

Also, the particular pattern used to alternate polarity of the acquiredNMR echo signals will depend on the order in which the phase encodingtraverses k-space. In the EPI pulse sequence of the preferredembodiment, k-space is traversed monotonically and the NMR echo signalsare alternated in polarity as they are received. However, if k-space istraversed in another order the pattern of NMR echo signals inversion maybe different. The important factor is that the NMR echo signals that arestored in the NMR data set alternate in polarity along the phaseencoding direction at the maximum rate possible.

It should also be apparent that other means can be used to alternate thepolarity of the NMR echo signals as they are received. The importantfactor is that this inversion occur before the introduction of the dclevel that produces the baseline error to be corrected.

I claim:
 1. A method for eliminating baseline error artifacts in NMRimages produced during a scan using an ultrafast pulse sequence, thesteps comprising:performing an ultrafast pulse sequence in which aseries of separately phase encoded NMR echo signals are produced inresponse to transverse magnetization produced by a single RF excitationpulse; acquiring the series of separately phase encoded NMR echo signalsand inverting the polarity of alternate phase encoded ones of theseparately phase encoded NMR echo signals as they are acquired; storingan NMR data set comprised of successive phase encoded ones of theseparately phase encoded NMR echo signals acquired during the scan;inverting the polarity of alternate phase encoded ones of the separatelyphase encoded NMR echo signals stored in the NMR data set; andreconstructing an image by performing a Fourier transformation on theNMR data set.
 2. The method as recited in claim 1 in which the scan iscompleted by performing a single ultrafast pulse sequence.
 3. The methodas recited in claim 1 in which the ultrafast pulse sequence is anecho-planar pulse sequence.
 4. The method as recited in claim 3 in whichthe separately phase encoded NMR echo signals scan a set of phaseencoding values in monotonic order.
 5. The method as recited in claim 1in which the series of separately phase encoded NMR echo signals areacquired with a receiver that mixes each NMR echo signal with areference frequency signal, and the polarity of alternate phase encodedones of the separately phase encoded NMR echo signals is inverted byinverting the polarity of the reference frequency signal.
 6. The methodas recited in claim 5 in which the polarity of the reference frequencysignal is inverted by shifting its phase 180°.